AUG translation initiation codon coding methionine is needed on translating a messenger RNA (mRNA) by use of a ribosome to synthesize a protein. The reason is that a special tRNA molecule dedicated to translation initiation, in the form of translation initiation methionine tRNA serving to define the initiation codon, is indispensable for initiation of translation. Therefore, in the protein synthesis system utilizing the conventional recombination DNA technique, the amino terminus of a protein, polypeptide and the like, which is produced via ribosome, always leads to methionine. Accordingly, the synthesis of the protein initiating from an arbitrary amino acid had been impossible. Recently, however, a method was published that permits synthesizing a protein having an amino acid other than methionine at the amino terminus (Toku Hyo 2000-517186). This method utilizes a DNA coding a mutant initiator tRNA containing an anticodon corresponding to an amino acid other than methionine.
On the other hand, in the translation initiation via the intergenic region-internal ribosome entry site (IGR-IRES) of an insect virus, the translation initiation point is determined by forming higher-order structure of IGR-IRES per se, i.e., the translation is initiated from glutamine (Ref. 2) or alanine (Refs. 3, 4, 5, and 7) without depending on the AUG translation initiation codon. About 40 bases at 3′ end of IGR-IRES had been reported (Ref 2), while the overall structure of IGR-IRES was not known. Previous reports (Refs. 1 and 6) had published a result showing that a partial sequence of the coding region of a coat protein gene of each virus locating downstream of IRES is also involved in the translation initiation, so that the translation initiation from the amino acid other than methionine, alanine, and glutamine had been thought to be impossible.